This is the 301st most frequent Finnish word.
päivänä
"Päivänä" means "on (a) day" in English.
The word 'päivänä' is in the essive case, specifying a particular day during which the action took place.
Tuona päivänä satoi vettä koko ajan.
On that day, it rained all the time.
The word 'päivänä' is used in the essive case, denoting the specific day of the birthday during which the action occurred.
Ystäväni syntymäpäivänä leivoimme ison kakun.
On my friend's birthday, we baked a big cake.
The word 'päivänä' in this sentence is in the essive case, referring to 'today' as a time reference for ongoing remembrance.